Re: [Buildroot] [RFC PATCH 1/2] support/scripts/genimage.sh: support creating a bmap image

["Gero Schwäricke via buildroot" <buildroot@buildroot.org>]

On Tue, May 21, 2024 at 07:44:24PM +0200, Yann E. MORIN wrote:
> Michael, All,
> 
> On 2024-05-21 11:28 +0200, Michael Nazzareno Trimarchi spake thusly:
> > On Tue, May 21, 2024 at 7:25 AM Yann E. MORIN <yann.morin.1998@free.fr> wrote:
> > > I think the question by Thomas was whether, given an input sparse file,
> > > genimage will keep the generated image sparse in the same places the
> > > input file was (only shifted by the offset that input file was copied
> > > at in the image).
> [--SNIP--]
> > Seems I did not understand the question. My idea on the topic is how
> > we can generate
> > images that we can easily flash. Now I understand the question of
> > Thomas and thank you.
> > 
> > Yann can I ask how we can use an image generated as you describe before?
> 
>     $ make pc_x86_64_efi_defconfig
>     $ make
>     $ ls -sl output/images/{rootfs.ext2,disk.img}
> 
> I tried a bit earlier, and the genimage file is indeed sparse, but I
> could not find a tool that would report the list of holes/non-holes in a
> file, and their offset, e.g. something like:
> 
>                 size    offset
>     non-hole    2MiB    @0
>     hole        1MiB    @2MiB
>     non-hole    16MiB   @3MiB
>  and so on...
> 
> So I could not validate whether the hole/non-hole structure is the same
> in the generated disk.img and the rootfs.ext2. If you have any idea how
> to do that, I'm all eyes.

Hi Yann,

iiuc, the bmaptool mainly uses the FIEMAP ioctl. I found this tool [1], 
which prints the FIEMAP values.

Here's a quick test I did:

$ truncate -s 1G /tmp/image.raw
$ ./fiemap /tmp/image.raw
File /tmp/image.raw has 0 extents:
#       Logical          Physical         Length           Flags

$ stat /tmp/image.raw 
  File: /tmp/image.raw
  Size: 1073741824      Blocks: 0          IO Block: 4096   regular file
Device: 10305h/66309d   Inode: 25692108    Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/gschwaer)   Gid: ( 1000/gschwaer)
Access: 2024-05-22 17:09:41.463441946 +0200
Modify: 2024-05-22 17:09:41.463441946 +0200
Change: 2024-05-22 17:09:41.463441946 +0200
 Birth: -
$ echo test >> /tmp/image.raw 
$ ./fiemap /tmp/image.raw
File /tmp/image.raw has 1 extents:
#       Logical          Physical         Length           Flags
0:      0000000040000000 0000006208224000 0000000000001000 0001
$ stat /tmp/image.raw 
  File: /tmp/image.raw
  Size: 1073741829      Blocks: 8          IO Block: 4096   regular file
Device: 10305h/66309d   Inode: 25692114    Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/gschwaer)   Gid: ( 1000/gschwaer)
Access: 2024-05-22 16:54:48.729576604 +0200
Modify: 2024-05-22 16:59:24.905638708 +0200
Change: 2024-05-22 16:59:24.905638708 +0200
 Birth: -

You can see the allocated section starting at logical 0x40000000 (1G).
Without the `echo` line, the extents list is empty, suggesting the file
has zero allocated blocks. `stat` also hints that the file is sparse,
by the small number of blocks (why 8? idk). I think this can be used to
verify the behavior of genimage.

[1] https://github.com/ColinIanKing/fiemap

Best,
Gero

> 
> Regards,
> Yann E. MORIN.
> 
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